A liquid crystal display (LCD) device has become remarkably widespread for use as an image display device for a personal computer and other various monitors. Generally, a liquid crystal display device is configured in such a manner that a backlight unit is provided as a planar light source for illumination, disposed on a backside of a liquid crystal display panel, and so that a liquid crystal surface having a specified area is irradiated on the whole with even brightness, thus enabling visualization of an image formed on the liquid crystal surface.
The backlight unit employs a hot-cathode or cold-cathode fluorescent lamp as a light source. Since light from the fluorescent lamp is linear light, this linear light must be converted into planar light for irradiating the entire surface of a liquid crystal display panel. For this purpose, heretofore, two systems of backlight units have been employed: a direct backlight unit; and a side (edge-light type) backlight unit. A direct backlight unit is configured in such a manner that a fluorescent lamp is placed immediately under a liquid crystal display panel, and a dimmer plate and a diffusion plate are disposed thereon. By contrast, a side backlight unit is configured in such a manner that one or two fluorescent lamps are disposed on one or two sides of a light guide plate made of transparent resin. Light made incident onto the light guide plate is then directed towards the liquid crystal display panel by a reflection structure provided on a front, back or both back and front of the light guide plate, thus even planar light is given thereto by use of light diffusion. Since a side backlight unit can be made thinner than the direct backlight unit, it is suitable for use as a display device in a portable device such as a notebook computer.
The following is a description of the principle of light emission of a side backlight unit. Light incident onto the light guide plate from the lamp travels through the light guide plate while making total internal reflection resulting from a difference in refractive indices (dielectric constants) between the material constituting the light guide plate and air. On at least one surface of the light guide plate, which is the back thereof in general, a reflection structure is provided for promoting the emission of light from the light guide plate. The total reflection with the air interface is disordered when the light traveling through the light guide plate strikes the reflection structure, and when the light is emitted from the front of the light guide plate. Note that, with reference to the light guide plate, the front signifies the surface of the light guide plate facing towards the liquid crystal display panel, and the back signifies the surface of the light guide plate opposite the front. Moreover, the surface from which the light is emitted is referred to as an emitting surface. Accordingly, the front includes the emitting surface. Usually the reflection structure is formed on the back of the light guide plate, and the light incident onto the light guide plate from the light source is emitted from the emitting surface of the light guide plate by the reflection structure. In order to allow an even luminance distribution at the emitting surface, a diffusion pattern is provided on the reflection structure that is less dense on the lamp side and grades to a higher density away from the lamp.
However, although the luminance distribution is made even, in prior art LCD devices using side backlight units, a problem occurs in that bright lines are generated at periodic positions from the lamp. Moreover, as the size of screens has increased, the distance from the screen to the lamp has decreased, and the bright lines have become increasingly conspicuous.
FIG. 8 is a drawing illustrating the generation of the bright lines. As shown in FIG. 8, particularly in the vicinity of a lamp R, bright lines K are generated on the screen. The bright lines K are portions having luminance higher than a periphery thereof lying parallel to the lamp R in the shape of a line.
The cause of the generation of the bright lines K has been pursued by those skilled in the art. As a result, a plurality of generation mechanisms for the bright lines K, particularly due to structural factors in the vicinity of the lamp R, have been clarified, and countermeasures for individual causes have been taken. However, the current situation is that the generation of the bright lines K has not yet been completely eliminated, and this problem remains to be solved.